Method for coating metallic interconnect of solid oxide fuel cell

ABSTRACT

Disclosed is a method for coating a metallic interconnect for a solid oxide fuel cell (SOFC), the method including the steps of: carrying out pre-treatment for removing impurities adhered on a surface of the metallic interconnect; and carrying out pulse plating with cobalt as an anode, and the metallic interconnect as a cathode, in which an average current density (I a ) is set in a room-temperature cobalt plating solution, and a maximum current density (I p ), a current-on time (T on ) and a current-off time (T off ) are adjusted based on I a =I p ×T on /(T on +T off ). Through the disclosed method, it is possible to obtain a metallic interconnect having a coating surface which has a high electrical conductivity and a high chrome volatilization inhibiting property and can minimize the occurrence of micro-cracks and micro-pores, thereby improving the performance of the SOFC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid oxide fuel cell, and moreparticularly to a coating method for forming a protective coating on thesurface of a metallic interconnect that interconnects unit cells andcollects current of a stack.

2. Description of the Prior Art

As power demands show a tendency to gradually increase according to arecent industrial development and economic growth, environmentalproblems, including air pollution and earth shock, have seriously arisenby the use of fossil fuels (such as petroleum, or coal) required forpower production. Especially, since the exhaust of carbon dioxide by theuse of fossil fuels is pointed out as a main factor of global warmingand various kinds of environmental pollution, the development of solarlight/heat energy, bio energy, wind energy, and hydrogen energy, asclean energy sources substituting for the fossil fuels, is beingactively conducted.

From among such clean energy sources, research on the field of fuelcells using a hydrogen fuel is active. A fuel cell technology isconsidered as a future electricity generation technology because a fuelcell does not exhaust pollutants in electricity generation, and has anadvantage in that it does not require a site for a power plant, a powertransmission facility, or a substation.

The fuel cell is divided into a phosphoric acid fuel cell (PAFC), amolten carbonate fuel cell (MCFC), a solid acid oxide fuel cell (SOFC),a solid polymer electrolyte fuel cell (a polymer electrolyte fuel cell(PEFC) or a proton exchange membrane fuel cell (PEMFC)), according tothe type of electrolyte. Herein, the phosphoric acid fuel cell has anoperating temperature of about 200° C., the molten carbonate fuel cellhas about 650° C., the solid oxide fuel cell has about 1000° C., and thesolid polymer electrolyte fuel cell has an operating temperature around80° C.

The SOFC, from among the cells, employs a solid oxide having oxygen ionconductivity as an electrolyte. Thus, the SOFC has an advantage in thatit has the highest efficiency as a fuel cell, can improve the efficiencyby up to 85%, due to inclusion of the heat generated by cogenerationwith a gas turbine, and can use various fuels. Also, since theelectrolyte for the SOFC is in a solid state, there is no loss in theelectrolyte and thus no need to supplement the electrolyte. Besides,there is no need to use a noble metal catalyst, and it is easy to supplya fuel through direct internal reforming.

The output performance of a unit cell of such an SOFC is reduced byvarious factors, such as polarization loss. Also, when a plurality ofunit cells of the SOFC are layered between a metallic interconnect, theoutput performance is influenced by the contact resistance between themetallic interconnect and the cells.

In a fuel cell, the metallic interconnect mainly performs a role ofelectrically interconnecting cells of a cell stack, and preventingsupplied gases within cells from mixing with each other, and is referredto as a bipolar plate or a separator.

At present, as a material of a metallic interconnect for the SOFC, astainless steel, such as STS430, and STS444, is used. Also, a newlydeveloped Crofer 22 APU may be used. However, by these materials, it isvery difficult to achieve the durability of up to 40000 hours requiredfor commercialization, and thus there is need to develop a novel alloyand to research the application of protective coating on the surface ofa conventional material.

A material for protective coating on the metallic interconnect for anSOFC may include various kinds of metal materials, such asPerovskite-type ceramic materials based on Lanthanum chromite (LaCrO₃)having a high electrical conductivity at a high temperature, spinel-typeceramic materials based on (Mn,Co)304 which is known to have a hightemperature conductivity and a high chrome volatilization inhibitingproperty, or transition elements forming a spinel structure (e.g.,manganese•cobalt•nickel•copper•chrome). Especially, examples of a methodfor forming a protective coating of transition metals forming the spinelstructure, from among the above materials, include sputtering,slurry-spraying, electrodeposition, chemical vapor deposition, or thelike.

The electrodeposition, from among recently used methods, is consideredas a method appropriate for future mass production of the metallicinterconnect for an SOFC due to its simple equipment and low cost. Oneof electrodeposition methods is a DC plating method.

In coating by using the DC plating method, a high current density isrequired to be applied to obtain a densified coating layer with fineparticles. However, at a higher current density than a predeterminedlimitation, plating ions around a to-be-coated substrate are depleted bymass transfer limiting conditions, thereby causing concentrationpolarization. Accordingly, there is a problem in that a plated surfaceis non-uniform and a densified coating layer cannot be formed. Also, incobalt protective coating using a DC plating method, micro-cracks andmicro-pores may be formed within a coating layer due to coarse particlesof the coating layer. Such micro-cracks and micro-pores cause someproblems, including peeling of an oxide film on a metallic interconnectsurface, and pollution of a cathode, by volatilizing chrome (Cr(IV)) gasfrom the metallic interconnect during the operation of the SOFC, andthus operates as a factor inhibiting an electrochemical reaction of theSOFC.

Therefore, research for establishing a technology of protective coatingof a metallic interconnect for an SOFC by using electrodeposition (whichhas not been attempted) and establishing plating conditions withimproved protection features is required.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and the presentinvention provides a method for coating a metallic interconnect for asolid oxide fuel cell (SOFC), which can minimize the occurrence ofmicro-cracks and micro-pores in a coating layer by formation ofdensified protective coating.

In accordance with an aspect of the present invention, there is provideda method for coating a metallic interconnect for a solid oxide fuel cell(SOFC), the method including the steps of: carrying out pre-treatmentfor removing impurities adhered on a surface of the metallicinterconnect; and carrying out pulse plating with cobalt as an anode,and the metallic interconnect as a cathode, in which an average currentdensity (I_(a)) is set in a room-temperature cobalt plating solution,and a maximum current density (I_(p)), a current-on time (T_(on)) and acurrent-off time (T_(off)) are adjusted based on a Mathematical Formula1 described below.

I _(a) =I _(p) ×T _(on)/(T _(on) +T _(off))  Mathematical Formula 1

Herein, the pulse plating may be carried out under a condition of thecurrent-on time (T_(on)) of 0.002˜0.005 seconds, the current-off time(T_(off)) of 0.005˜0.008 seconds, the maximum current density (I_(p)) of100˜250 mA/cm², and the average current density (I_(a)) of 30˜50 mA/cm².

Also, the step of carrying out the pre-treatment may include the stepsof polishing the surface of the metallic interconnect by silicon carbideabrasive paper, washing off the impurities on the surface of themetallic interconnect by 10% NaOH aqueous solution and acetone, removinga surface fine scale of the metallic interconnect by 10% HCl solution,and carrying out pickling for 30 to 60 seconds.

The size of the anode may be 1˜1.5 times larger than that of thecathode, and an interval between the anode and the cathode may be 1˜2times larger than a width of the cathode.

Also, the plating solution may employ a Watts bath of cobalt sulfate(CoSO₄.7H₂O) and cobalt chloride (CoCl₂.6H₂O), in which pH is maintainedfrom 2 to 4 by a cobalt hydroxide aqueous solution or a dilutedhydrochloric acid solution.

Also, after the pulse plating, heat-treatment may be further carried outin a 800° C. reducing atmosphere (10% H₂+90% N₂) for 2˜20 hours toenhance a binding force between the metallic interconnect and a platedcoating layer.

Through the method according to the present invention, it is possible toobtain a metallic interconnect having a coating surface which has a highelectrical conductivity and a high chrome volatilization inhibitingproperty and can minimize the occurrence of micro-cracks andmicro-pores, thereby improving the performance of the SOFC.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a conceptual view illustrating a pulse plating device forcoating a metallic interconnect for a solid oxide fuel cell (SOFC),according to the present invention;

FIG. 2 shows the shape of pulse current applied to pulse platingaccording to the present invention, compared to the shape of DC current;

FIG. 3 shows photographs of plated coating surfaces after pulse platingaccording to the present invention;

FIG. 4 shows photographs of the surfaces of plated coating layers, whichillustrates the results of pulse plating according to the change of aduty ratio of (T_(on)/T_(off)) in the present invention;

FIG. 5 shows photographs of the surfaces of plated coating layers, whichillustrates the results of pulse plating according to the change of amaximum current density (I_(p)) and an average current density (I_(a))in the present invention;

FIG. 6 shows photographs of the results of pulse plating according tothe present invention under the conditions of FIG. 5, which wereobserved by AFM (Atomic Force Microscope);

FIG. 7 shows photographs of the surfaces of metallic interconnect testsamples, in which the test samples were coated by conventional DCplating and pulse plating of the present invention with the samequantity of electric charge;

FIG. 8 shows photographs of cross sections of metallic interconnect testsamples, in which the test samples were coated by conventional DCplating and pulse plating of the present invention with a samethickness;

FIG. 9 shows photographs of cross sections of metallic interconnect testsamples, in which the test samples coated with cobalt by pulse platingaccording to the present invention were subjected to oxidationevaluation in a 800° C. oxidizing atmosphere, and the extent ofvolatilization of chrome from the test samples was observed;

FIG. 10 shows the result of X-ray diffraction analysis, which wascarried out to find out a phase change of the cobalt protective coatingby high temperature oxidation, after pulse plating on a metallicinterconnect according to the present invention; and

FIG. 11 shows the measurement results of electrical conductivity ofmaterials for a metallic interconnect, in a state where the materialswere coated with cobalt by pulse plating according to the presentinvention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, an exemplary embodiment of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood, however, that the following embodiment is illustrative only,and the scope of the present invention is not limited thereto. Also,those skilled in the art will appreciate that various modifications,additions and substitutions are possible.

In the operation of a solid oxide fuel cell (SOFC), a chrome oxide scaleformed on the surface of a metallic interconnect at a high temperaturecauses some problems, such as reduction of a sealing property by peelingof the scale, pollution of a cathode by volatilization of chrome fromthe scale, or the like. These problems reduce output performance andlong-term durability of the SOFC.

Accordingly, in the method according to the present invention, on thesurface of a metallic interconnect, cobalt, one of transition metals forforming a spinel layer with a high electrical conductivity and a highchrome volatilization inhibiting property, is coated through pulseplating. This improves the electrical conductivity in a high temperatureoxidizing atmosphere, and inhibits the pollution of a cathode byinhibiting growth and volatilization of chrome oxide, so as to improvethe output performance and long-term durability of the SOFC.

Before pulse plating is carried out on a metallic interconnect,pre-treatment for removing impurities adhered on the surface of themetallic interconnect is performed. Hereinafter, the pre-treatment willbe described in detail. First, the surface of the metallic interconnectis polished by using silicon carbide (SiC) abrasive paper (preferably,abrasive paper with roughness of #100˜2000). Second, 10% NaOH aqueoussolution and acetone are used to wash off the surface impurities of themetallic interconnect. Third, 10% HCl solution is used to remove a finescale on the surface of the metallic interconnect, and then pickling iscarried out for 30 to 60 seconds.

On the metallic interconnect which has been subjected to the abovedescribed pre-treatment, pulse plating is carried out by using a pulseplating device.

FIG. 1 is a conceptual view illustrating a pulse plating device forcoating a metallic interconnect for an SOFC, according to the presentinvention. As shown, within a plating bath 1 containing a platingsolution L, an anode 2 and a cathode 3 are immersed, and the anode 2 andthe cathode 3 are connected to a pulse generating device 4.

The plating solution is a room-temperature cobalt plating solution thatemploys a Watts bath of cobalt sulfate (CoSO₄.7H₂O) and cobalt chloride(CoCl₂.6H₂O), in which the pH is maintained from 2 to 4 by a cobalthydroxide aqueous solution or a diluted hydrochloric acid solution. Thecobalt chloride and the cobalt chloride improve current efficiency andreduce pit density so as to obtain a uniform plated surface. To theWatts bath, boric acid is preferably added so as to provide buffering ofpH, and to reduce stress. The acidity (pH) of a plating solution ismaintained between 2˜4, so that a uniform plated layer can be obtained.

As the anode 2, a mesh type cobalt plate is used, and as the cathode 3,a stainless steel (STS430, STS444, Crofer 22 APU), that is, a materialof a metallic interconnect for an SOFC, is used. In general, in anelectrochemical reaction, it is known that a potential difference and alocal current density over an interface between a plating solution andan electrode change along the surface of the electrode. The currentdensity is higher at a protruded portion of an electrode, or is higherat edges when the interval between an anode and a cathode is larger thanthe width of an electrode. In this case, an ‘edge effect’ whichincreases the plating thickness of edge portions, compared to a centerportion, may occur, thereby having a bad influence on the uniformity ofthe plating thickness. Accordingly, in order to obtain a uniform platingthickness, it is preferable that electrodes have a similar size, and thewidth of an electrode is same or similar to the interval betweenelectrodes. Thus, in the present invention, the size of the anode 2 is1˜1.5 times larger than that of the cathode 3, and the interval betweenthe anode 2 and the cathode 3 is 1˜2 times larger than the width of thecathode 3.

FIG. 2 shows the shape of pulse current applied to pulse plating,compared to the shape of DC current. As shown, the shape of pulsecurrent is shown as a pulse waveform by a current-on time (T_(on)) and acurrent-off time (T_(off)) based on the average current density (I_(a)).

Meanwhile, the pulse generating device 4 is used to set the averagecurrent density (I_(a)) in the room-temperature cobalt plating solution,and a maximum current density (I_(p)), a current-on time (T_(on)) and acurrent-off time (T_(off)) are adjusted based on the MathematicalFormula 1 described below to carry out pulse plating.

I _(a) =I _(p) ×T _(on)/(T _(on) +T _(off))  Mathematical Formula 1

Through the test, the inventor of the present invention found thatduring pulse plating, it is possible to apply a higher current densitythan an average current density of DC plating by adjusting thecurrent-on time (T_(on)) and the current-off time (T_(off)), and toapply a maximum current density (I_(p)) according to an increase in thecurrent-off time (T_(off)), and this makes it possible to obtain finecoating particles.

In other words, when the pulse plating was carried out under thecondition where (T_(on)+T_(off)) was changed from(0.0005+0.0005)˜(5.0+5.0) seconds, the maximum current density (I_(p))was fixed within a range of 100˜250 mA/cm², and the average currentdensity (I_(a)) was fixed within a range of 30˜50 mA/cm², as can be seenfrom the photograph of the coating surface shown in FIG. 3, the particlesize shows a tendency to increase according to an increase in(T_(on)+T_(off)). Especially, under the condition where (T_(on)+T_(off))was (0.005+0.005) seconds, a fine-particle coating film was formed.

As described above, after pulse plating according to the presentinvention, when heat-treatment is carried out in a 800° C. reducingatmosphere (10% H₂+90% N₂) for 2˜20 hours, the binding force between themetallic interconnect and the plated coating layer can be enhanced.

FIG. 4 shows photographs of the surfaces of plated coating layers, whichillustrates the results of pulse plating according to the change of aduty ratio of (T_(on)/T_(off)) in the present invention. In other words,under the condition where the average current density (I_(a)) had afixed value of 50 mA/cm², and the maximum current density (I_(p)) wasadjusted between 62.5˜250 mA/cm² while the duty ratio of(T_(on)/T_(off)) was changed to 20, 50, and 80%, the surface particlesof the plated coating layer show a tendency to become fine according toa decrease in T_(on), and an increase in T_(off). The reason for this isthat plating ions can be sufficiently re-diffused around the cathodeduring T_(off).

FIG. 5 shows photographs of the surfaces of plated coating layers, whichillustrates the results of pulse plating according to the change of amaximum current density (I_(p)) and an average current density (I_(a))in the present invention. Under the condition where a duty ratio of(T_(on)/T_(off)) was fixed at 20%, the maximum current density (I_(p))was changed to 250, 200, 150 mA/cm², and the average current density(I_(a)) was changed to 50, 40, 30 mA/cm², it is determined that athrowing power of the surface becomes better as the maximum currentdensity increases. In general, since a much higher maximum currentdensity can be applied in pulse plating, compared to that in DC plating,it is possible to obtain a high nucleation rate of plating particles,thereby forming fine plating particles.

FIG. 6 shows photographs of the results of pulse plating according tothe present invention under the conditions of FIG. 5, which wereobserved by AFM (Atomic Force Microscope), and it can be seen that onthe whole the surface roughness of the plated layer is low.

In brief, through the test results of FIGS. 4 to 6, it can be determinedthat the particles on a plated surface are fine and uniform, and have ahigh throwing power under the conditions of a current-on time(T_(on))=0.002˜0.005 seconds, a current-off time (T_(off))=0.005˜0.008seconds, a maximum current density (I_(p))=100˜250 mA/cm², and anaverage current density (I_(a))=30˜50 mA/cm².

Meanwhile, hereinafter, the case where pulse plating according to thepresent invention is carried out to coat a metallic interconnect for anSOFC, will be described, compared to conventional DC plating, withreference to FIGS. 7 and 8.

FIG. 7 shows photographs of the surfaces of metallic interconnect testsamples, in which the test samples were coated by conventional DCplating and pulse plating of the present invention with the samequantity of electric charge (current×time), and it can be seen that thetest sample coated by the pulse plating has a better throwing power,compared to the test sample coated by the DC plating.

FIG. 8 shows photographs of cross sections of metallic interconnect testsamples, in which the test samples were coated by DC plating and pulseplating with a same thickness. It can be seen that pores exist inpatches on the cross-section coated by DC plating, while pores hardlyexist on the cross-section coated by pulse plating.

FIG. 9 shows photographs of cross sections of metallic interconnect testsamples, in which the test samples coated with cobalt by pulse platingaccording to the present invention were subjected to oxidationevaluation in a 800° C. oxidizing atmosphere and the extent ofvolatilization of chrome from the test samples was observed. It can beseen that the volatilization of chrome was effectively inhibited.

FIG. 10 shows the result of X-ray diffraction analysis, which wascarried out to find out a phase change of the cobalt protective coatingon a metallic interconnect test sample by high temperature oxidation.FIG. 10 a indicates the result just after plating, and FIG. 10 bindicates the result of X-ray diffraction analysis after hightemperature oxidation for 1000 hours. Through the analysis, it wasdetermined that a spinel phase having a high electrical conductivity anda high chrome volatilization inhibiting property was formed, whichincludes cobalt.

FIG. 11 shows the measurement results of electrical conductivity ofmaterials (STS430 and STS444) for a metallic interconnect, in a statewhere the materials were coated with cobalt by pulse plating accordingto the present invention. In other words, when the area specificresistance was measured while the materials were maintained in a 800° C.oxidizing atmosphere for 1000 hours, it was observed that a low value of11˜31 mΩcm² was maintained for about 1000 hours during the evaluation ofhigh temperature electrical conductivity.

As described above, through the pulse plating according to the presentinvention, it is possible to form a spinel phase containing cobalt onthe surface of a metallic interconnect for an SOFC, thereby providing acoating layer having a high electrical conductivity and a high chromevolatilization inhibiting property. Also, it is possible to obtaindensified protective coating in which the occurrence of micro-cracks andmicro-pores is minimized.

Although an exemplary embodiment of the present invention has beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for coating a metallic interconnect for a solid oxide fuelcell (SOFC), the method comprising the steps of: carrying outpre-treatment for removing impurities adhered on a surface of themetallic interconnect; and carrying out pulse plating with cobalt as ananode, and the metallic interconnect as a cathode, in which an averagecurrent density (I_(a)) is set in a room-temperature cobalt platingsolution, and a maximum current density (I_(p)), a current-on time(T_(on)), and a current-off time (T_(off)) are adjusted based on aMathematical Formula 1 described below.I _(a) =I _(p) ×T _(on)/(T _(on) +T _(off))  [Mathematical Formula 1] 2.The method as claimed in claim 1, wherein the pulse plating is carriedout under a condition of the current-on time (T_(on)) of 0.002˜0.005seconds, the current-off time (T_(off)) of 0.005˜0.008 seconds, themaximum current density (I_(p)) of 100˜250 mA/cm², and the averagecurrent density (I_(a)) of 30˜50 mA/cm².
 3. The method as claimed inclaim 1, wherein the step of carrying out the pre-treatment comprisesthe steps of polishing the surface of the metallic interconnect bysilicon carbide abrasive paper, washing off the impurities on thesurface of the metallic interconnect by 10% NaOH aqueous solution andacetone, removing a fine scale on the surface of the metallicinterconnect by 10% HCl solution, and carrying out pickling for 30 to 60seconds.
 4. The method as claimed in claim 1, wherein a size of theanode is 1˜1.5 times larger than a size of the cathode, and an intervalbetween the anode and the cathode is 1˜2 times larger than a width ofthe cathode.
 5. The method as claimed in claim 1, wherein the platingsolution employs a Watts bath of cobalt sulfate (CoSO₄.7H₂O) and cobaltchloride (CoCl₂.6H₂O) with pH of 2 to 4, the pH being maintained by acobalt hydroxide aqueous solution or a diluted hydrochloric acidsolution.
 6. The method as claimed in claim 1, wherein after the pulseplating, heat-treatment is carried out in a 800° C. reducing atmosphere(10% H₂+90% N₂) for 2˜20 hours to enhance a binding force between themetallic interconnect and a plated coating layer.